Hello, World!

This chapter walks you through creating your first [Psy Smart Contract Language] program.

Creating a New Program

Create a new project:

dargo new hello_world

This creates a new directory with a basic project structure. Navigate to the project and edit the src/main.psy file:

// input: 2,3
// output: 5
fn main(a: Felt, b: Felt) -> Felt {
    assert(a < b, "a should be less than b");
    assert_eq(b - a, 1, "b - a should equal 1");
    return a + b;
}

This simple program:

• Takes two Felt parameters (a and b)
• Asserts that a is less than b
• Asserts that the difference is exactly 1
• Returns the sum of a and b

Compiling

Compile the program using the compiler:

dargo compile

You will see a target directory created with the compiled output. The Psy compiler generates DPN opcodes and circuit data for each function.

[
    {
        "name": "main",
        "method_id": 680917438,
        "circuit_inputs": [
            0,
            1
        ],
        "circuit_outputs": [],
        "state_commands": [],
        "state_command_resolution_indices": [],
        "assertions": [
            {
                "left": 4294967296,
                "right": 4294967297,
                "message": "x != y"
            }
        ],
        "definitions": [
            {
                "data_type": 0,
                "index": 0,
                "op_type": 0,
                "inputs": [
                    0
                ]
            },
            {
                "data_type": 0,
                "index": 1,
                "op_type": 0,
                "inputs": [
                    1
                ]
            },
            {
                "data_type": 1,
                "index": 0,
                "op_type": 13,
                "inputs": [
                    0,
                    1
                ]
            },
            {
                "data_type": 1,
                "index": 1,
                "op_type": 2,
                "inputs": [
                    1
                ]
            }
        ]
    }
]

Running

Execute the program with test inputs:

dargo execute --program-dir . --entry-path src/main.psy --parameters 2,3

This runs the main function with the parameters 2 and 3. You should see the output result_vm: [5] as the result.